1. Field of the Invention
The present invention relates generally to passive optical networks (PONs) and, more specifically, to avoiding interference in PONs in which radio frequency video signals are overlayed on packet data signals.
2. Description of the Related Art
Telecommunication service providers are using their networks to deliver an ever-broadening array of services to their subscribers' or customers' homes, businesses or other premises. Whereas once a digital subscriber line (DSL) or cable television line was used solely to provide subscribers with World Wide Web and e-mail access, today's service providers wish to bundle Internet service with voice (telephony) and video (television) services. Such bundled Internet, voice and video service is sometimes colloquially referred to as “triple play” service. The desire to bundle video service along with voice and Internet access has driven telecommunication service providers to turn increasingly to optical fiber-based technologies, such as the passive optical network (PON). Fully optical telecommunications service networks, which some have referred to as “fiber-to-the-premises” (FTTP), are increasingly being developed and deployed.
Most digital telecommunications networks (i.e., networks that facilitate the communication of data, voice, video, etc., between parties or between a content distribution service and subscribers) typically comprise active components, such as repeaters, relays and other such devices that consume power, in the path between a central office (or exchange, as its sometimes referred to) and a subscriber. In addition to requiring power, active components are subject to failure and performance degradation over time, and may require significant periodic maintenance. The passive optical network (PON) has been developed to overcome some of these deficiencies. The essence of a PON is that nothing but optical fiber and passive components are found in the path between the central office and subscribers. A single fiber can run from the central office to a passive splitter located near a group of subscribers, such as a neighborhood or office complex, and individual fibers can run from the splitter to individual subscribers or sub-groups of subscribers.
The International Telecommunications Union (ITU) and the Institute of Electrical and Electronics Engineers (IEEE) are two standards-making bodies currently developing PON standards. The ITU has adopted recommendations of the Full Service Access Networks (FSAN) organization, including G983.x, a specification sometimes referred to as “broadband PON” (BPON), and G984.x, a specification sometimes referred to as “gigabit PON” (GPON). The IEEE has also adopted Ethernet-based (i.e., IEEE 802.3-based) PON standards referred to as “Ethernet PON” (EPON) and “gigabit EPON” (GEPON). These standards and recommendations are well known to persons skilled in the art to which the invention relates and are therefore not described in further detail in this patent specification.
In accordance with these standards, a PON comprises an optical line terminal (OLT) (also known as optical line terminator), which is typically located at the central office, and a number of optical network terminators (ONTs) (also known as optical network terminals and optical network units), each located at the subscriber's premises (e.g., home, office building, etc.), with optical fiber and one or more splitters between the OLT and ONTs. In the downstream direction, i.e., data transmitted from the OLT (e.g., located at the central office) to an ONT (e.g., located at a subscriber's premises), the data units are broadcast from the OLT to all of the ONTs on the PON, and an ONT can select the data to receive by matching the address embedded in the data units to a previously provisioned or learned address. In other words, an ONT only “listens” to data units having a matching address. Thus, the OLT can transmit data to a particular or selected ONT by addressing it to that ONT. In GPON, the address typically comprises a GPON Encapsulation Method (GEM) Port-ID. In the upstream direction, i.e., data transmitted from an ONT to the OLT, the data units are time-domain multiplexed.
Two methods for delivering video services over a PON are being explored. In one method, the video is digitally encoded using the same transport mechanism and protocol (e.g., Internet Protocol) and transmitted in the same manner as any other digital data transmitted on the PON. In the other method, radio frequency (RF) signals that represent video information are overlayed, using optical wavelength division multiplexing (WDM) techniques, on the digital packet data signals that represent voice, Internet communications and all such other information. In other words, at the service provider's central office or PON head end, the video signal is coupled onto the fiber at a wavelength that is different from the wavelength at which data traffic from the OLT is transmitted on the fiber. Downstream, at each subscriber's ONT, the RF video signal and digital packet data are separated by a triplexer or similar device. In a case in which the RF video signal is transmitted in analog form, the separated video signal can be sent directly to a television set. In a case in which the RF video signal is transmitted in digital form, a set-top box first decodes the signals.
A periodic signal in the optical data stream can interfere with the video signal, causing interference lines on the television screen and thereby degrading the subscriber's viewing experience. This type of interference is due to an effect known as Raman interference or Raman scattering. Transmission of a stream of uniform packets can result in visible degradation of an RF video signal due to Raman interference. As illustrated in FIG. 2, the OLT conventionally transmits non-informational or placeholder “idle” frames bearing some predetermined data pattern to maintain ONT synchronization when no information is to be transmitted, i.e., when the data source is idle. The predetermined pattern is commonly referred to as an idle pattern. (Although the IEEE Ethernet standard (802.3ah) defines idle patterns for Ethernet packets, a simple pattern of alternating ones and zeroes is shown in FIG. 2 for purposes of illustration.) All idle frames have the same length (“L” in FIG. 2). Transmission of a stream of idle frames is problematic in a PON with video overlay because the repetitive idle pattern contains frequency components that overlap the frequencies of video signals.
Solutions for minimizing Raman interference during idle frame transmissions include substituting random data for the idle pattern. Nevertheless, the noise reduction that these solutions provide is not always sufficient to make the effects of Raman interference unnoticeable to a television viewer.
It would be desirable to provide a method and apparatus that reduces Raman interference in a PON with RF video overlay to a level that is unnoticeable to a television viewer. The present invention addresses these problems and deficiencies and others in the manner described below.